Process for producing hot rolled three layer steel products from continuously cast hollow tubes

ABSTRACT

The invention relates to a process for producing three-layer rolled products by rolling composite ingots. The ingots are produced by casting bars according to the continuous casting process and guiding them, when issuing from the mould, along a curved path at first downwardly and then upwardly until a height at which the liquid core of the bar cannot ascend any further so that when casting and transportation is continued a hollow bar is obtained; said hollow bar then is cut into pieces which are permitted to solidify and then are poured full with a metal of a composition different from the composition of the bar. This process permits a simple and low-cost production of sheets clad with layers which, compared to the overall thickness of the sheets, are very thin.

[451 July 3,1973

[ PROCESS FOR PRODUCING HOT ROLLED THREE LAYER STEEL PRODUCTS FROM CONTINUOUSLY CAST HOLLOW TUBES [75] Inventors: Giswalt Veitl; Ernst Bachner, both of Linz, Austria [73] Assignee: Vereinigte Osterreichische Eisenund Stahlwerke Aktiengesellschaft, Linz, Austria 22 Filed: Sept. 17,1971

[211 App1.No.: 181,627

[30] Foreign Application Priority Data Sept. 22, 1970 Austria 8546/70 56] References Cited UNITED STATES PATENTS 10/1967 Tarmann 164/85 1/1969 Neumann 164/86 3,421,571 1/1969 Webber et al. 164/86 3,482,259 12/1969 Schwarz 164/94 X FOREIGN PATENTS OR APPLICATIONS 1,281,115 10/1968 Germany 164/85 Primary Examiner-R. Spencer Annear Attorney-Granville M. Brumbaugh et a1.

[57] ABSTRACT The invention relates to a process for producing threelayer rolled products by rolling composite ingots. The ingots are produced by casting bars according to the continuous casting process and guiding them, when issuing from the mould, along a curved path at first downwardly and then upwardly until a height at which the liquid core of the bar cannot ascend any further so that when casting and transportation is continued a hollow bar is obtained; said hollow bar then is cut into pieces which are permitted to solidify and then are poured full with a metal of a composition different from the composition of the bar. This process permits a simple and low-cost production of sheets clad with layers which, compared to the overall thickness of the sheets, are very thin.

5 Claims, 3 Drawing Figures PROCESS FOR PRODUCING HOT ROLLED THREE LAYER STEEL PRODUCTS FROM CONTINUOUSLY CAST HOLLOW TUBES The invention relates to a process for producing rolled products comprising three layers, the medium layer having a composition different from the composition of two outer layers, particularly for producing metal sheets. Composite ingots comprising three layers are produced by casting hot liquid metals into a mould. The outer layers of the composite ingot, which are formed from a metal of the one composition, are formed in the shape of a hollow body open on both ends which is permitted to solidify. Subsequently, this body which represents the form is poured full with a metal of another composition for the formation of the medium layer of the composite ingot.

In the steel processing industry there is an increasing demand of sheets, jobbing sheets and plates clad at both sides, and of clad sticks and wires. The main demand of material clad at both sides is for sheets and jobbing sheets provided with stainless coatings which are used in particular in the production of automobile parts liable to corrosion, such as exhaust parts, floor panels, door frames and the like. Also, sheets clad at both sides are used for household and kitchen gadgets. As corrosion resistant coating or cladding material suitably chromium steel with about l7 percent chromium, or chromium-nickel steel with about 18 percent chromium and about 9 percent nickel is used. In these rust and corrosion resistant coatings the layer thickness on each side should amount to 0.1 to 0.5 mm, depending on what the material is used for; thus, compared to the overall thickness of the sheet, the coatings are very thin; e.g., in automobile sheets having a thickness from about 0.7 to 1mm, the cladding layers have a thickness of about 0.1 mm.

Multi-layer sheets are also used for agricultural machinery, for example as plough shares. These sheets are called three layer sheets and comprise as a rule two hard outer layers, for example made of structural steel having a tensile strength of 70 kp/mn, and a softer, tough middle layer, for example made of structural steel having a tensile strength of 37 kp/mn for particular purposes also a hard medium layer and softer outer layers may be provided.

Clad sheets are produced as a rule by roll cladding, the materials to be connected being welded by the rolling pressure at hot rolling temperature. The surfaces of the materials to be welded together have to be metallically blank and above all there must be no oxide film, in particular no chromium oxide films. For the production of two sheets clad on one side, as a rule four sheets of the sheet bar type are laid one above the other and connected to form a cladding sandwich which is to be separated again after rolling, the two outer sheets being made of the basic material and the two inner sheets which are to be connected with the other sheets being made of the cladding material. In that process the margins of the basic material sheets project beyond the margins of the two medium sheets and are connected with each other by welding steel brackets. thereto; then margin welding seams are applied around the margins of the sandwich. The air included within the sandwich causes the formation of at least local oxide films when the sandwich is heated to rolling welding temperature, which makes bonding impossible. In order to avoid this oxide film formation the sandwich has to be flushed heating and rolling. Although this device enables to avoid any waste due to the clad sheets showing separation areas caused by oxide films and thus a very high yield is obtained, the production of clad sheets by applying the rolling welding cladding process, which in modified form is also applicable for sheets which are clad on both sides, is very expensive and complicated so that a large-scale use of clad sheets for mass articles has not been possible so far.

Clad sheets may also be produced by rolling a composite steel ingot. In the British patent specification No. l,l02,l49 a process for producing such multi-layer composite steel ingots is described which are particularly used for the production of three layer sheets (plough share sheets). In that process a slightly conical mould of customary design is used which is placed on a bottom pouring plate; a partition wall is inserted in its middle so that two hollow mould spaces are obtained. At first steel is cast into the two hollow mould spaces until they are filled up. Several minutes later the partition wall is loosened and removed and the hollow space thus created is filled with liquid steel of another quality which welds with the steel first cast in. The composite steel ingot thus produced which comprises three layers of about the same thickness then is rolled in the rolling mill. For casting technological reasons it is, however, not possible to produce with this process a composite ingot in which the outer layers compared to the total thickness are very thin. The moulds used in this process and the partition walls to be inserted into them usually have a temperature of less than C prior to casting and they have a considerable wall thickness so that their heat absorption is very great and the liquid steel solidifies rapidly. It is therefore not possible to produce a metallurgically faultless composite ingot with layers of for example 1 m width, 5 to 10 cm thickness and 2 to 5 m height; according to experience the thickness of a layer of the composite steel ingot, just as the thickness of an ingot cast in customary manner in the mould, should not be essentially less than 200 mm; furthermore for reasons which are connected with the formation of piping and segregation during solidification the ingot height is limited at about 2 to 2.5 m. For casting technicological reasons only relatively thick composite steel ingots can be produced, the use of which is not economical for the production of sheets and jobbing sheets with extremely thin coatings, since hot rolling has to be carried out in two heats in order to obtain the necessary reduction of thickness. Further, it is a disadvantage that when the sheet production program is changed to another total thickness and/or coating thickness, the thickness of the partition wall to be inserted in the mould has to be changed, which is rather cumbersome, and moreover such changes are possible only within certain limits because of the first mentioned metallurgical difficulties.

It is also known to produce composite steel ingots by permitting the marginal layer of a cast ingot to solidify at first and then pouring in a melt of another chemical composition whereby the liquid core is displacedand the solidified ingot ring is poured full with the metal of the other composition. In this known process, however, a mixing of the two hot liquid metals cannot be avoided for it is known from casting practice that not only a mechanical mixing but also a mixing by diffusion of the individual elements into the deeper situated metal layers takes place. In the production of ingots with an outer layer of rust and/or acid corrosion resistant material, which, as is well known must have a very low carbon content, the use of this known casting method would lead to an enrichment of carbon in the steel present in the ladle and it is then no longer possible to cast another ingot of this impuritied steel in which the marginal layer is made of an alloyed steel poor in carbon. Thus, the process is accompanied by severe metallurgical disadvantages and also it cannot be economically employed because of the low yields.

ln continuous casting, relatively small casting cross sections may be obtained without difficulties; with slabs, the width of the cast products usually amounts to from 800 to 2,000 mm and the thickness to from 150 to 300 mm so that such castings can be hot rolled in one heat. A further advantage of continuous casting resides in the high yields. A process for the continuous casting of tubes is known (see the Austrian patent specifications Nos. 253 153, 263 242 and 263 243 and Bergund Huettenmaennische Monatshefte (ll 1) 1966, p. 317/320), in which the wall thickness of the tubes can be varied within wide limits by appropriate adjustment and control of the main casting conditions of the continuous casting plant. in this process for easting hollow bars as is the case with the casting of full bars liquid melt is guided to the upper end of a water cooled continuous casting mould and the bar issuing from the mould is guided along a curved path which descends at first and ascends then to a height at which the liquid part of the bar cannot ascend any further so that when casting and transportation of the bar iscontinued the liquid metal in the interior of this bar extends only to said height and the bar ascending beyond this height is formed to be hollow. For carrying out this process advantageously a continuous casting plant with an areshaped mould is used and the bar is guided along an arc until above the height of the casting level. Contrary to customary processes for the continuous casting of hollow bodies in which a mould with a cooled thorn is used which is inserted in its middle, the hollow bodies produced according to this process have an absolutely dense wall. This advantage is explained by the fact that when melts solidify most porous areas or hollow spaces originate where solidifying fronts meet. But there is only one solidifying front which shifts from the bar surface into the interior and then, onwards from the place to which the liquid melt extends in the upwardly guided bar, forms the inner surface of the hollow bar. ln casting tests it has shown that the inner surface of the ho]- low bar formed in this manner is surprisingly smooth, so that a hollow bar thus produced represents a good starting material for the production of seamless tubes. In the production of particularly thin walled hollow bars it is, perhaps, necessary when the casting speed is changed to cool the bar leaving the mould only over a short distance with splashing water and to provide it with a heat insulation in the remaining part of the curved path which is run through by the bar having a liquid core, so that a further heat dissipation is prevented. By an appropriate coordination of the casting speed and the bar cooling the wall thickness of the hollow bar may thus be varied in wide limits. Thus, also thin walled casting products with uniform wall thickness can be produced metallurgically faultlessly.

The invention pursues the task of creating a simple and cheap process for the production of rolled products comprising three layers by rolling composite steel ingots in which the thickness of the individual layers may easily be changed and in which particularly the thickness of the outer layers may be kept relatively small in relation to the total thickness of the ingot.

In a process of the kind defind in the introduction the invention resides in that the hollow body to be poured full with said metal of another composition is produced by continuous casting of a steel melt in particular by means of a continuous casting mould which is arranged to be substantially vertical, the liquid melt being supplied to the upper end of the continuous casting mould and the bar issuing from the mould being guided along a curved path falling at first and ascending then beyond a height at which the liquid part of the bar cannot rise any further so that when casting and transportation of the bar is continued, the liquid steel in its interior reaches only until said height and the bar ascending beyond said height is hollow, whereupon the endless bar is severed into hollow bar pieces open on both ends.

In the process according to the invention thus any mixing of the hot liquid metals is avoided. Moreover the process according to the invention permits a continuous casting operation with a maximum yield. Furthermore, the wall thickness may easily be regulated.

The hollow steel body is advantageously poured full in the red hot condition, preferably immediately after its solidification, its mean temperature amounting to more than 700C and its inner temperature ranging between 900 and 1,200C.

Preferably the hollow body, prior to being poured full from above or from below, is placed on a floor plate or on a bottom pouring plate, respectively, it being supported at least at two lateral faces lying opposite each other. According to the temperature of the hollow body and the relation of its wall thickness to the thickness of the composite steel ingot the outer surface of the composite ingot may be cooled after pouring full is terminated.

These features of the invention and further details may be derived from the following examples and the accompanying drawings.

FIG. 1 of the drawings show a schematic vertical section of a continuous casting plant for producing a hollow cast bar and FIGS. 2 and 3 show pouring stands for filling sections of the hollow bar.

EXAMPLE 1 In a circular arch type continuous casting plant (FIG. 1) for slabs with a bar guide comprising a semicircular first part 2 with a radius R which is joined by a second part 3 which is straight and vertical and extends beyond the casting level 4 of the steel 5 in the mould 6, a hollow bar 7 is cast having an outer cross section of 1,200 mm X300 mm and a wall thickness of 60 mm. The casting speed and the bar cooling rate are controlled so that the diameter of the liquid bar core 8 amounts to about 180 mm at the end of the semi-circular bar guide 2. The hollow bar is cast from a steelwith a composition comprising 0.22 C, 0.40 Si, 0.50 Mn, 17.00 Cr and 2.00 Ni, the temperature measured in the tundish 9 above the continuous casting mould 6 amounting to l,530 C and the casting speed to 2.5 m/min.

In the area of the vertical part of the bar guide 3 extending up beyond the casting level 4, the hollow bar 7 is cut into several bar pieces (7' in FIG. 2 each) having a length of 4,000 mm by means of two powder cutting means 10 which are synchronously moved together with the hollow bar 7. The direction of movement of the cutting means 10 is indicated by a double arrow. Each of these bar pieces 7 is immediately thereafter set on a floor plate 11 which is arranged next to the continuous casting plant 1, the support surface 12 being sealed by asbestos strips or asbestos cords 13 in customary manner so that the hollow bar pieces 7' subsequently may be filled up from above by casting (FIG. 2). For this purpose a rimming deep drawing steel 14 is used comprising 0.07 C and 0.50 Mn, the temperature of which measured in the casting ladle 15 amounts to 1,550 C. The temperature of the hollow bar pieces at the beginning of filling up amounts, at the inside, to about l,l00 C and at the outside to about 900 C. Thus a faultless welding of the soft deep drawing steel 14 with the chromium-nickel steel of the hollow body 7 is made possible. In order to prevent bulging of the hollow bar pieces or hollow bodies, respectively, by the ferrostatic pressure occurring during filling up, a supporting means is fixed on the floor plate 11, surrounding the wall of the hollow bodies; this supporting means may, for example, comprise a closed casting body 15 or a roller corset 16 and is necessary in partcular for supporting the broad sides of the hollow bodies. This supporting means may also be water cooled so that the composite steel ingot is solidified more rapidly; for example, cooling spraying nozzles 17 may be used.

The solidified composite steel ingot of 1,200 mm width, 300 mm thickness and 4,000 mm height, after an ultrasound test for inner faults, is abraded by means of a slab abrading machine, by which process a layer of about 5 to 10 mm thickness is removed all around.

Then the ingot is heated to l,l80 C in a continuous type furnace and rolled out on a break-down stand and a six-stand broad strip street; in the break-down stand seven passes are carried out so as to produce a pre-strip of 25 mm thickness; at the end of the hot broad strip street the thickness of the hot strip amounts to 3 mm and the issuing temperature amounts to 880 C. The hot strip is divided by flying shears into sheet plates which are piled at a temperature of 750 C and then annealed at a temperature of 950 C. After the annealing process the sheet plates are cut in a circular knife shearing street, straightened and pickled. The final size of the sheet plate amounts to l,l00 X2,500 X3 mm, the thickness of the cladding layer on both sides amounting to 0.5 mm; with an initial thickness of 60 mm and an abrasion of 10 mm on each side, this corresponds to a hundrcdfold shaping of the cladding layers.

EXAMPLE 2 After the continuous casting mould 6 has been exchanged, on the same circular arch continuous casting plant 1 for slabs, a hollow bar is cast having an outer cross section of 1,600 mm 250 mm and a wall thickness of 60 mm. The hollow bar is cast from a steel with a composition comprising 0.06 C, 0.40 Si, 1.30 Mn, l8.00 Cr and 9.00 Ni, the temperature measured in the tundish 9 above the continuous casting mould 6 amounting to l,490 C and the casting speed to 2.5 m/min. The hollow bar is cut into several bar pieces having a length of 4,000 mm each which are immediately thereafter set on a bottom pouring plate 18 (FIG. 3). At the beginning of casting, the temperature of the hollow bar piece 19 amounts at the inside to 1,000 C and at the outside to about 800 C. A killed steel comprising 0.10 C, 0.30 Si, 0.40 Mn and 0.01 A1 and having a temperature, measured in the casting ladle, of 1,570 C is poured into the hollow bar piece 19. For casting a funnel 22 is used. In order to keep piping as small as possible, the steel is covered immediately after termination of pouring by an exothermal piping powder.

The solidified composite steel ingot of 1,600 mm width, 250 mm thickness and 4,000 mm height then is similarly treated as explained in Example 1: after an ultrasound test, abrasion of its surface and heating in the continuous type furnace to l,220 C the composite steel ingot is rolled on a breakdown stand by seven passes so as to produce a prestrip of 25 mm thickness which leaves the hot broad-strip street with an issuing temperature of 900 C and a thickness of 4 mm. The hot strip is reeled at a temperature range of 550 to 600 C and finally pickled in a pickling plant designed for high-grade steels. Then local faults are abraded off. The pickled and abraded hot strip is cold rolled, then annealed at 950 C under protective gas, then dressed, straightened and sheared to plates having a final size of 1,500 X2,000 Xl mm. The thickness of the cladding layers amounts to 0.166 mm on each side; with an ini tial thickness of 60 mm and an abrasion of 10 mm on each side of the composite ingot, this corresponds to a threee hundred-fold shaping of the cladding layers.

Obviously the process according to the invention may also be used for producing three-layer composite steel ingots for three layer sheets in which as described at the beginning the ratio of the thicknesses of the layers is substantially l l 1. In this case only the casting and cooling conditions in the continuous casting plant have to be changed so that hollow bars with a greater wall thickness can be produced. Further, also hollow billets may be produced by continuous casting which filled up in similar manner serve as starting product for merchant bar or wire.

1. A process for producing three-layer rolled metal products having two outer layers of a first composition and an inner layer of a second composition, the steps comprising:

continuously casting a liquid metal melt of a first composition downwardly from a casting mould to form an endless metal bar,

allowing the bar to solidify from the outside while keeping the inner core of the bar substantially liquid,

guiding the cast bar in a curved path until the bar is being moved in an upwardly direction,

moving the bar upwardly until the solidifying outer surface extends above the liquid inner core, forming an endless hollow cast bar,

cutting the hollow cast bar into sections, I

positioning the hollow sections in supporting means,

filling each of the hollow sections with a liquid metal melt of a second composition while the mean temperature of the hollow sections is at least 700C 900C to 1,200C, and

subjecting the filled sections to subsequent rolling and shaping operations to produce three-layered rolled metal products.

2. The process set forth in claim 1 wherein the hollow sections are positioned vertically and are filled from the 5 bottom. 3. The process set forth in claim 1 wherein the hollo sections are positioned vertically and are filled from the the step of cooling the filled sections.

P0405?) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.- 37 13, 5 Dated ly 3, 1973 Inventor) Giswalt Veitl and Ernst Bachner It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 1, lines +1 and +3, "K /m should be --kp/mm Col. 2 line 1, "insert" should read --inert-'-;

Col, Q line 5, "2 each) should be --2) each"; and

Col. 5 line 31, "partcnlar" should read --'pa.rticular--.

Signed and sealed this 19th day of March 1974.

:SEAL) fittest:

EDWARD M.FLETCHER,JR. C. MARSHALL DANN ittesting Officer Commissioner of Patents 

2. The process set forth in claim 1 wherein the hollow sections are positioned vertically and are filled from the bottom.
 3. The process set forth in claim 1 wherein the hollow sections are positioned vertically and are filled from the top.
 4. The process set forth in claim 1 wherein the supporting means support each of the hollow sections at least at two positions opposite each other.
 5. The process set forth in claim 1 further comprising the step of cooling the filled sections. 